(1) Field of the Invention
The invention relates to a method for estimating the components of a force torsor applied to a bearing comprising a fixed member, a rotary member and at least one row of rolling bodies arranged between said members in order to allow for their relative rotation.
In particular, the invention applies to motor vehicle wheel bearings, the fixed ring being intended to be attached to the frame of the vehicle, the wheel being intended to be rotatable by the intermediary of the rotating ring, and two rows of balls being provided between said rings.
(2) Prior Art
In many applications, in particular in relation with assistance and safety systems such as ABS or ESP, it is necessary to determine the forces that are applied to the interface between the wheel and the roadway on which said wheel rotates.
To do so, it is known to carry out measurements on the tyre or on the frame. However, the measurement on the tyre poses major problems linked to the instrumentation of a rotating part, in particular relative to:
the power supply of the sensors, because a battery does not have a sufficient lifespan;
the transmission of the signal, which must take place with a frequency and a reliability that are sufficient to be able to act on the assistance and safety systems. Even if solutions do exist, this wireless transmission becomes further complicated as the number of pieces of information to transmit is substantial and in that this information is to be received in real time;
the calculation of the forces in a fixed marking. It is in particular necessary to constantly know the position of the rotating marking in relation to the fixed marking.
Concerning measurement on the frame, this is made difficult by the distribution of the forces between the different members that connect the wheel to said frame (tie rod, A arm, shock absorber, etc.).
Consequently, as proposed in particular in FR-2 812 356, the fixed ring, which is the first linking member between the wheel and the frame, can be advantageously used as a support for the determination of the forces that are exerted at the interface between the wheel and the roadway during the displacements of the vehicle.
In particular, the determination of the forces is carried out by measuring the deformations of the fixed ring which are induced by the passing of the rolling bodies. Indeed, the amplitude of these deformations is representative of the forces transmitted by the bearing. However, the difficulty is to extract from the deformation measurements the information relative to the components of the force torsor that is applied on the bearing.
In order to attempt to resolve this problem, WO-2005/040745 presents an algorithm of which a portion is based on a neural network in order to form the link between the measurements of deformations and the components of the torsor. The disadvantage with this type of method, referred to as “black box”, is its robustness with regards to situations which were not taken into account during the learning phase. Another blocking point is not taking the preload into account which is a factor of the first degree in the behaviour of the bearing, and all the more so in that this preload can vary over time with the change of the temperature but varies especially between different bearings in terms of manufacturing dispersions.
Moreover, WO-2005/008204 presents a method based on a physical model resulting from a finite element calculation, wherein:
the ball/ring contact forces are calculated using the resolution of said physical model according to the deformation measurements carried out; and
the force torsor is calculated by adding the various contact forces together.
The first disadvantage of this method is the number of unknowns. Indeed, the system of equations to be solved comprises as unknowns all of the contact forces.
The second disadvantage stems from the frequency processing needed to elaborate an estimation of the components of the torsor. Consequently, this method does not provide a good level of performance in terms of bandwidth since it is based on a frequency expression of the behaviour of the bearing.